wire1.art

一个很小和天线计算程序

SUBJECT:  Wire antennas - part one

    Dollar for dollar, the wire antenna is an Amateur Radio 
Operators (AROs) best bet when it comes to inexpensive antennas.
In the lower bands, 160, 80, and 40m, the wire antenna is 
nearly the only means for most AROs to work those bands.

    Wire antennas perform from great to poor depending on many
factors even when the antenna is cut for the frequency desired.
I'll discuss some of my findings with wire antennas in this ten 
part series. Since most of the wire antennas are published in 
numerous antenna books, the specifics of most wire antennas 
will be left to the reader to investigate for the band they 
wish to operate in.  Instead, I will discuss some helpful 
hints, findings, and misgivings about wire antennas that
the ARO can use in overcoming the difficulties encountered 
with their first wire antennas and some that the old pros may 
find usefull.

    I will be discussing the advantages and disadvantages of 
wire antennas on different bands, wire diameter, ground height, 
matching, multi-band wire antennas, and a few other odds and 
ends.

    ---------------------------------------------------------

    First, let's look at the capture area of a wire antenna cut 
as a dipole.  We can examine this at 10m for ease of calculations 
and understanding.

    Many hams use a #12 or #10 wire when constructing their wire 
antennas. It's cheap and easy to obtain.  Some wire antennas 
come as kits and use strained wire verses the solid conductor 
many hams use in their home construction.  Looking at the 
diameter of the wire, whether strained or not, it can be easily 
seen that a dipole made from aluminum tubing has a greater 
capture area, without picking up a calculator to find out the
difference.   Larger diameter elements produce greater 
bandwidths, thus a wire dipole on 10m would have a limited 
bandwidth compared to it's aluminum tubing counterpart.

    How does one overcome this?  Through trial and error, 
and a tight budget when I first got into Ham radio, I found 
that cutting the wire antenna slightly shorter that the 
designed frequency and adding some tubing to the end of the 
dipole, I was able to expand the bandwidth of the wire 
antenna upto three times the orignal value.

    It doesn't take much tubing or a large diameter to 
accomplish this. I used two of these units (of course, one 
on each end).  There are two parts to the extention.  Use 
a 12-in x 1/2-in diameter tube with an 8-in x 3/8-in tube 
telescoping inside the former.  By placing a vertical cut 
into the 12-in section, a clamp can be used to lock the 
two tubes together.  Now you have tunable end pieces.  
Run the support rope through the tubing and attach to 
the wire then secure the wire to the 1/2-in end of the 
tubing.  A good electrical contact is a must.  The rope 
will support the tunable end piece. 

    The end pieces are also much greater in diameter than 
the wire, adding to the capture area.  Also they provide a 
tunable method for zeroing the dipole to the frequency 
desired or changing the frequency of the dipole later.  
At 10m, I would recommend an aluminum tubing dipole and 
save this technique for the 30m and below antennas.  For 
the lower frequency wire dipoles and inverted vees, the 
tubing can be made much longer for better results.  There 
is a fine line between weight and performance that needs 
to looked into when using this method and it's a function 
of the antenna support and support lines.

    I stumbled across this technique while putting around 
with a 40m inverted vee.  I just could not get the bandwidth 
and SWR right.  It was either too high for the resonate 
frequency or too low no matter what the calculator thought.
The SWR was 2:1 and I wanted an antenna that did not require 
a tuner for my solid state radio.  I grew tired of soldering 
and cutting wire and decided to add the tubing extenders.
Not only did I get the Inverted Vee on target, 7.15 MHz, but 
I covered the entire 40m band and the MARS frequency I was 
required to attend on. The SWR was below 1.5:1 across the 
entire band also!  :-)

    It worked so well I decided to bring it to field day, 
1991.  The antenna worked very well and without a tuner.  
If you have any questions about the modification, let me 
know on HAM_TECH.



-WS